This application claims the priority benefit of Taiwan application Ser. No. 88111661, filed Jul. 9, 1999.
1. Field of Invention
The present invention relates to a folding mirror. More particularly, the present invention relates to a folding mirror used for a pick-up head of an optical recording/reproducing device.
2. Description of Related Art
Generally speaking, an optical recording/reproducing device can use a optical disc, such as a compact disc-read only memory (CD-ROM) or a digital versatile disc (DVD), to record a large amount of data therein, and can reproduce data stored in the optical disc. As the multimedia technology highly developes, the optical disc capable of storing a large amount of video and audio data is more popular. Furthermore, because of highly developed DVD technology, the capacitance dramatically increases to 4.7 gigabyte (GB) from 650 megabyte (MB) of the traditional CD-ROM disc, and even increases to 15 GB in the feature.
The optical recording/reproducing device has an optical pick-up head used for reading data stored in an optical disc or writing data into an optical disc. A laser beam emitted from a light source passes through an objective lens within the optical pick-up head and then focuses at an optical disc. Therefore, light spots are formed on an information layer of the optical disc. The laser beam is then reflected by the information layer and received by a photo detector. By this, the data stored in the optical disc are read out.
However, as the capacitance and storage density of the optical disc increase, the quality of the spots formed on the information layer is easily affected by the tilt of the optical disc. The deformed sports are usually in an asymmetric coma shape, known as coma aberration. When the coma aberration occurs, the reflected laser beam by the optical disc is burred, which causes signals read by the optical pick-up head to be distorted. Therefore, it is an important subject how to compensate the affection of the coma aberrations due to the tilt of the optical disc and how to improve the quality of the laser beam spots for reading data.
Accordingly, several methods for compensating errors due to the coma aberrations and improving the quality of the laser beam spots are proposed. For example, the U.S. patent (U.S. Pat. No. 5,523,989), issued to Matsushita Corp., Japan, 1996, discloses a disc driver having an additional lens for compensating the coma aberration. When the optical disc tilts, a servo mechanism drives comatic lenses to compensate the coma aberration errors. However, in addition to the comatic lenses, this method needs an actuator to drive the comatic lenses. Therefore, the overall assembly tolerance range for the disc driver becomes tightened.
The Pioneer company utilizes a liquid crystal phase modulator to compensate coma aberration error due to the tilt of the optical disc. The proposed method needs additional driver circuits and special designed driving voltages to modulate phase deviations of different spatial positions to compensate the coma aberration errors. However, this method can not normally function for high-speed disc drivers because the special resolution is limited and the response time of the liquid crystal is slow.
In addition, the U.S. patent (U.S. Pat. No. 5,723,054), issued to Eastman Kodak Corp., U.S., 1998, discloses that sensors are assembled to an actuator for detecting and determining the tilt status of an optical disc. According to a detected disc tilt error signal, a base used for mounting the actuator moves angularly to compensate the coma aberrations. However, a complex mechanism is required to achieve the compensation and the complexity and volume of the whole system increase.
According to the conventional methods discussed above, additional optical compensation lens and/or actuator must be added within the optical path of the pick-up head for compensating the coma aberrations due to the tilt of the optical disc. Therefore, the volume of the pick-up head and the difficulty for assembling the optical pick-up head increase.
The invention provides a folding mirror device, which can be applied to an optical pick-up head of an optical recording/reproducing device. The folding mirror device comprises a folding mirror, a piezoelectric film and electrodes attached to the surfaces of the piezoelectric film. Accordingly, the present invention does not need to design new additional optical elements or actuators to modify the optical pick-up head. The present invention utilizes the piezoelectric film accompanied with the electrodes having particular patterns and voltages applied to the piezoelectric film to deform surface of the folding mirror attached to the piezoelectric film. Due to the deformation of the folding mirror, the coma aberrations are compensated and therefore the signal quality for reading data from the optical disc is improved.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a folding mirror structure for compensating the coma aberrations. The folding mirror structure comprises a folding mirror for reflecting a laser beam, a piezoelectric film attached to the surface of the folding mirror. The piezoelectric film further comprises electrodes attached to the surfaces of the piezoelectric film, which each of the electrodes has particular pattern thereon.
Accordingly, by properly adjusting the applied external electric field to electrodes attached to the piezoelectric film, the piezoelectric film deforms which causes the folding mirror deforms to be capable of compensating the coma aberrations.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.